Far Out! Making Crystals Ripple with Light

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A beam of light can make waves in crystals, and those waves can
be "tuned" — a phenomenon that might open up new technological
possibilities, researchers say.

At the University of California, San Diego, physicists led by
Dimitri Basov and Siyuan Dai fired a beam of infrared light at a
tiny crystal of boron nitride. They focused the beam on the tip
of an atomic force microscope. An
atomic force microscope probes surfaces at the scale of atoms
and molecules with a needle at the end of an arm, like that on a
vinyl record player. The
microscope transferred the momentum from the light to the
crystal.

The light generated ripples — waves — in the boron nitride. The
waves, called phonon polaritons, had wavelengths as short as
those of ultraviolet light, about 300-400 nanometers, or
billionths of a meter. [ Magnificent
Microphotography: 50 Tiny Wonders ]

"A wave on the surface of water is the closest analogy," Basov
said in a statement. "You throw a stone and you launch concentric
waves that move outward. This is similar. Atoms are moving. The
triggering event is illumination with light."

A chemical used in cosmetics, boron nitride
(BN) is a van der Waals crystal, which means its atoms form
layers, stacked on top of one another and held together by forces
between molecules. By adjusting the wavelength of the light and
the number of layers of boron nitride, the researchers were able
to adjust the shape and size of the polaritons.

"The key novelty is that the wave properties can be tuned by
altering the number of atomic layers in a [boron nitride]
specimen," Basov told Live Science.

Since it's possible to control the size of the waves, it's also
possible to use the crystal to transmit information, in a way
similar to how light is used in radio communications. "You can
direct information where you want it at the nanoscale," Basov
said.

The ability to tune
polaritons also means one can control the flow of heat in a
material, since heat is just the movement of atoms and molecules
in a substance.

Control of waves could be important to building nanometer-size
circuits. Right now, information is transmitted between circuit
components with electrons. Light has all kinds of properties that
make it useful for transmitting data; for instance, it's fast.
But to use light waves to transmit information, a simple antenna
generally has to be at least half as large as the light
waves (this is why antennas for radios are as big as they
are). It's possible to make them shorter, but there are
trade-offs in efficiency. [ The
9 Biggest Unsolved Mysteries in Physics ]

Radio waves, in even the fastest networks, have wavelengths
measured in tenths of a millimeter. The infrared waves common in
TV
remotes are even smaller, just micrometers long. Even so,
that's thousands of times the size of typical computer circuits,
which are tens of nanometers across — they are simply too small
to use radio frequencies. (When you use a Wi-Fi network, the
radio signal is converted into electrons so the computer can
"hear" it, and requires an antenna — the Wi-Fi radio can be large
compared to a processor.)

Making the radio waves in the signal shorter isn't always an
option; such wavelengths eventually move from radio into the
visible light range, and that requires re-tooling the transmitter
and receiver. Also, how well waves transmit can be highly
dependent on the wavelength used and the environment they are in.
For example, longer radio waves bend around corners more easily
than visible light, which is why you don't need to be in the line
of sight of the local FM station.

The ability to transmit light-like waves in a solid substance
would mean that technologists would get many of the advantages of
light waves, without some of the problems of generating
ultra-short wavelength signals like the need for a
transmitter/receiver setup.

Smaller circuits also have a bigger problem radiating away heat.
Computers have fans to keep the processors cool, but using light
to control the temperature might mean future machines could
dispense with them.

The work started with experiments in graphene,
Barsov said. Graphene, which is made of carbon, also forms
single-molecule layers, and also can make polaritons in response
to light. The waves, however, don't last as long as they do with
boron nitride. "People thought boron nitride was just a bystander
material – we never thought it would be useful," Basov said.